Helical springs that are curved when unstressed are known from German Patents 1 505 616 and 3 743 450 for example. Springs of this type are employed to advantage in automotive-axle dashpots because they can be positioned with their curvature extensively compensating for undesirable friction and bending moments against the shock-absorber piston rod.
Curved helical springs have until now been manufactured only on cold-winding mechanism without mandrels as described for instance on pages 60 to 62 of Meissner and Wanke's Handbuch Federn, Berlin, VEB Verlag Technik, 1988. This procedure achieves the desired curvature without the expense of varying every other winding flight.
Helical springs can in one particular alternative to winding without using mandrels be manufactured hot on what are called coiling benches. They can be shaped by two parallel cylindrical rollers, one called the mandrel and the other the trainer. The training function can on the other hand be assumed by a computerized numerically controlled wire and rod feed as described on pages 185 and 186 of the Hoesch Hohenlimburg AG literature Warmgeformte Federn issued on the occasion of the 52nd IAA in Frankfurt-am-Main in 1987 and on pages 106 and 107 of the Stahlwerke Bruninghaus GmbH literature Technische Data Fahrzeugfedern issued on the occasion of the 45th IAA in Frankfurt-am-Main in 1973.
German 4 124 326 discloses a method of manufacturing curved helical springs on existing hot-coiling production lines by cooling or heating sections of the coil before or while it sets.
Reproducible industrial-scale manufacture of curved helical springs by this known method extensively depends on local flow boundaries, which are directly determined by the spring's temper strength and local temperatures.
In addition to the locally affected flow boundaries, stress that is precisely selected in accordance with the particular cross-section or coil section must be applied during the setting phase to achieve the desired curve. This is much too expensive, given contemporary tolerance demands.